I need extra outlets in my kitchen and in order to install a bigger junction box, I need to cut the glass tile that's already installed. Is it feasible and, if so, what's the most effective tool to use?
I agree with the above posts that a diamond-tipped blade is preferred. Dremel EZ545 shown (but many other options available)
Diamond tooling, without a doubt.
My personal preference runs to a dry-cut diamond blade on a small grinder (with a vacuum cleaner held in close proximity on the side dust spews out of) which I would describe as "quick, but can be messy." With a good vacuum carefully held, not TOO messy.
Given that you are cutting for junction boxes, you can overlap the cuts at the corner since the excess cut (if not excessive) will be hidden under the outlet cover plate. For other purposes or if you don't want those little bits cut, you can stop at the corner and finish with some other diamond cutting tool.
In theory (and perhaps in practice, just not my practice) you can also use an appropriate bit on a "cutout tool" (lightweight router mostly intended for drywall cutouts.)
To some extent your choice may depend on what tools you already own that you can get a diamond blade for or what tool you find most reasonable to purchase for the job, considering your other uses for the tool.